1. Field of the Invention
The present invention relates to an acoustic wave filter device for use in, for example, an RF stage of a cellular phone, and, more particularly, to an acoustic wave filter device that has a balance-unbalance conversion function.
2. Description of the Related Art
In order to reduce the sizes of cellular phones, surface acoustic wave filter devices are widely used in the RF stages of the cellular phones. When a surface acoustic wave filter device has a balance-unbalance conversion function, it is possible to omit a balun used for balance-unbalance conversion.
Further, in a band-pass filter used in the RF stage, the impedance at a side at which the band-pass filter is connected to an antenna terminal is often different from the input impedance of a differential amplifier that is connected in the stage subsequent to the band-pass filter. In general, the characteristic impedance of an antenna is 50Ω. On the other hand, the input impedance of the differential amplifier that is connected in the stage subsequent to the surface acoustic wave filter device often ranges from about 100Ω to several hundreds Ω. Thus, when a surface acoustic wave filter device has an impedance conversion function, there is no need for a separate circuit for impedance matching.
Therefore, for example, the following Japanese Unexamined Patent Application Publication No. 2002-290203 discloses a surface acoustic wave filter device that has a balance-unbalance conversion function and in which the ratio of the input impedance to the output impedance is 1:4. In this case, if the impedance of an antenna is 50Ω and the input impedance of a differential amplifier is 200Ω, impedance matching can be performed without separately providing a matching circuit.
However, the characteristic impedance of the differential amplifier is varied. If the characteristic impedance of the differential amplifier is much less than 200Ω, an impedance matching circuit has to be additionally provided when the surface acoustic wave filter device disclosed in Japanese Unexamined Patent Application Publication No. 2002-290203 is used. Thus, there is also demand for a surface acoustic wave filter device that has a balance-unbalance conversion function and in which the ratio of the characteristic impedance at an unbalanced terminal side to the characteristic impedance at a first and second balanced terminals side is much less than 1:4.
On the other hand, the following Japanese Unexamined Patent Application Publication No. 2002-300005 discloses a surface acoustic wave filter device that has a balance-unbalance conversion function and has input and output impedances that are substantially equal to each other. FIG. 9 is a schematic plan view that shows an electrode structure of the surface acoustic wave filter device disclosed in Japanese Unexamined Patent Application Publication No. 2002-300005.
In a surface acoustic wave filter device 200, an electrode structure shown in the drawing is formed between an unbalanced terminal 201 and first and second balanced terminals 202 and 203. In other words, a longitudinally coupled resonator type first surface acoustic wave filter 204 is connected between the unbalanced terminal 201 and the first balanced terminal 202. In addition, a longitudinally coupled resonator type second surface acoustic wave filter 205 is connected between the unbalanced terminal 201 and the second balanced terminal 203.
The surface acoustic wave filters 204 and 205 respectively have first to third IDTs 211 to 213 and 221 to 223 that are arranged in this order along a surface acoustic wave propagating direction in which a surface acoustic wave propagates. In addition, reflectors 214 and 215 and reflectors 224 and 225 are respectively arranged at both sides of the surface acoustic wave filters 204 and 205 in the surface acoustic wave propagating direction.
The first and third IDTs 211 and 213 have first and second divided IDT portions 211A and 211B, and 213A and 213B that are provided by dividing each of the first and third IDTs 211 and 213 into two portions in an electrode finger overlap width direction in which electric fingers overlap each other. First ends of the IDTs 211 and 213 are connected in common to the unbalanced terminal 201. Second ends of the IDTs 211 and 213 are connected to a ground potential. A first end of the second IDT 212 in the middle is connected to the ground potential, and a second end is connected to the first balanced terminal 202.
Similarly, in the second surface acoustic wave filter 205, the first and third IDTs 221 and 223 have first and second divided IDT portions 221A and 221B, and 223A and 223B that are provided by dividing each of the first and third IDTs 221 and 223 into two portions in the electrode finger overlap width direction. Then, first ends of the IDTs 221 and 223 are connected in common to the unbalanced terminal 203, and each of second ends is connected to the ground potential. A first end of the IDT 222 is connected to the ground potential, and a second end thereof is connected to the second balanced terminal 203.
Here, in the IDTs 211, 213, 221, and 223, the first and second divided IDT portions 211A and 211B to 223A and 223B, which are provided by dividing the IDTs 211, 213, 221, and 223 in the electrode finger overlap width direction, are connected in series with each other.
In the surface acoustic wave filter device 200, the impedance at a connection point 216 in FIG. 9 is about four times the impedance at the first balanced terminal 202. Similarly, the impedance at a connection point 226 is about four times the impedance at the second balanced terminal 203. In the entire surface acoustic wave filter device 200, the impedance at the unbalanced terminal 201 is substantially equal to the impedance at the first and second balanced terminals 202 and 203.
The surface acoustic wave filter device 200 disclosed in Japanese Unexamined Patent Application Publication No. 2002-300005 has a balance-unbalance conversion function. In addition, the input impedance and the output impedance can be substantially equal to each other. Thus, when the surface acoustic wave filter device 200 is used in the RF stage of a cellular phone, it is possible to omit an impedance matching circuit and to simplify the structure even when the input impedance of a differential amplifier that is connected in the subsequent stage is close to the characteristic impedance of an antenna.
However, in the surface acoustic wave filter device 200, at least the two surface acoustic wave filters 204 and 205 are connected in parallel with the unbalanced terminal 201. Thus, the overall size of the surface acoustic wave filter device 200 has to be large in the surface acoustic wave propagating direction because the first surface acoustic wave filter 204 and the second surface acoustic wave filter 205 are arranged on a piezoelectric substrate in the surface acoustic wave propagating direction. Therefore, it is difficult to reduce the size.